1. Field of the Invention
The present invention relates to a solid electrolytic capacitor. More particularly, it relates to a solid electrolytic capacitor comprising an organic semiconductor as a solid electrolyte.
2. Description of the Prior Arts
Recently, as electrical equipment circuits are digitized, a small capacitor having low impedance in a high frequency range and large capacity is increasingly required. Hitherto, as a capacitor for use in the high frequency range, a plastic film capacitor, a mica capacitor and a laminated ceramic capacitor are known and used. However, since the film capacitor and mica capacitor are large in size, it is difficult to increase their capacity. The laminated ceramic capacitor has deteriorated temperature characteristics, as it is miniaturized with increasing capacity, so that its production cost considerably increases. As a capacitor with large capacity, an aluminum dry electrolytic capacitor and an aluminum or tantalum solid electrolytic capacitor are known. These capacitors can be made to have large capacity since an anodic oxidation film which serves as a dielectric of the capacitor can be made very thin. However, since the anodic oxidation film tends to be easily damaged, an electrolyte to repair damage of the oxidation film should be provided between the film and a cathode. The aluminum wet electrolytic capacitor is prepared by winding a pair of etched aluminum foils for an anode and a cathode with inserting between them a separator made of paper impregnated with a liquid electrolyte. Therefore, its capacitance decreases and/or loss angle (tan .delta.) increases as time passes due to leakage or evaporation of the liquid electrolyte. Further, it has very poor high frequency characteristics and low temperature property because of electrolytic conductivity. The aluminum or tantalum solid electrolytic capacitor utilizes manganese dioxide as a solid electrolyte to overcome the above drawbacks of the aluminum wet electrolytic capacitor. The manganese dioxide solid electrolyte is prepared by dipping an anode element in an aqueous solution of manganese nitrate followed by pyrolysis at about 350.degree. C. Since this type capacitor comprises the solid electrolyte at high temperature, it does not suffer from leakage of the electrolyte or decrease of performance due to solidification of the electrolyte in a low temperature range, so that it has better frequency and temperature characteristics than a capacitor comprising a liquid electrolyte. However, its impedance or tan .delta. in a high frequency range is ten times or more larger than that of the laminated ceramic capacitor or the plastic film capacitor because of damage of the anodic oxidation film by pyrolysis of the manganese nitrate and large resistivity of manganese dioxide.
To solve the problems of the above described capacitors, it is proposed to use an organic semiconductor, namely 7,7,8,8-tetracyanoquinodimethane (hereinafter referred to as "TCNQ") complex as a solid electrolyte, since it has large conductivity and good anodic oxidation property. This organic semiconductor can be incorporated in the anodic oxidation film by dissolving it in an organic solvent or by melting it at high temperature and prevent damage of the oxidation film during pyrolysis for the formation of manganese dioxide. Since the TCNQ complex has large conductivity and good anodic oxidation property, it can provide a capacitor having improved high frequency characteristics and large capacity. For example, Japanese Patent Kokai Publication No. 17609/1983 discloses use of an organic semiconductor comprising N-n-propyl- or N-isopropyl-isoquinoline and TCNQ as a solid electrolyte in a capacitor. According to said invention, the TCNQ salt is impregnated in a wind type aluminum electrolytic capacitor by melting the TCNQ salt. Thereby, an aluminum capacitor having greatly improved frequency and temperature characteristics is produced since strong bonding is formed between the TCNQ salt and the anodic oxidation film and also the TCNQ salt has large conductivity. As the aforementioned Japanese Patent Publication discloses the use of organic semiconductor comprising the TCNQ salt as the solid electrolyte, a capacitor comprising the organic semicondutor has better frequency and temperature characteristics than the solid electrolytic capacitor comprising manganese dioxide since the TCNQ salt has larger conductivity and better anodic oxidation performance (repairing property). According to said Publication, TCNQ salt comprising a cation of isoquinoline the nitrogen atom of which has been quarternarized with alkyl groups is molten and impregnated in the anodic oxidation film.
The TCNQ salt comprising a cation of isoquinoline the nitrogen atom of which has been quarternarized with alkyl groups has varying melting properties and heat stability with the kind of alkyl groups. Further, since such salt has different conductivity and bonding property with the oxidation film which serves as a dielectric film, the characteristics of capacitor vary with the kind of alkyl groups. Although the TCNQ salt containing an isoamyl or n-butyl group achieves excellent characteristics of the capacitor, the following factors should be taken into consideration:
(1) Reliability after kept standing at high temperature.
(2) Achieved capacitance by electrolyte used.
(3) Generation of poisonous gas during pyrolysis.
Fluctuation of the capacitor characteristics after kept standing at high temperature includes decrease of capacity increase of tan .delta., increase of leakage current and the like. Quality assurance temperature of the conventional capacitor is usually 85.degree. C. and at most 105.degree. C.
The conventional dielectric film is etched to form micropores so as to increase capacitance of the capacitor. Since a solution type electrolyte can be well trapped by the micropores, it can achieve largest capacitance (the capacitance achieved by the liquid electrolyte is regarded as 100%). However, the TCNQ salt can achieve only 70 to 80 % capacitance.
In addition, the TCNQ salt has a cyano group, it will generate a poisonous cyanide gas at a temperature higher than its decomposition temperature.